1. Field of the Invention:
This invention relates to semiconductor devices and more particularly to an ion implant mask and cap during anneal for fabricating submicron gallium arsenide structures.
2. Description of the Prior Art:
In the prior art, fabrication of gallium arsenide structures may begin by applying an organic photoresist layer on the upper surface of a gallium arsenide substrate and patterning it in an appropriate manner to form, for example, a field effect transistor (FET) active layer mask. The next step is to ion implant impurities through the photoresist mask where there are windows or openings to form a doped region extending from the surface of the gallium arsenide substrate to a predetermined depth. The photoresist layer is subsequently removed and a capping layer is deposited over the gallium arsenide substrate.
The material of a capping layer may, for example, be silicon nitride, silicon oxide, phosphorus-doped silicon oxide or aluminum nitride. The purpose of the capping layer is to reduce the outgassing of arsenic from the gallium arsenide substrate when the ion implanted region is annealed. The ion implanted region is annealed by raising the gallium arsenide substrate to a high temperature such as 800.degree. C. to permit recrystallization of the gallium arsenide damaged by the ion implantation. During recrystallization, substitution of the ion implanted ions into the crystal lattices of the gallium arsenide material occurs. After the ion implanted region is annealed, a step also called activation, the capping layer is removed and further processing continues. This includes the formation of ohmic contacts defining drain and source and deposition of material suitable to form the gate of a field effect transistor.
Other materials suitable as a capping layer to prevent outgassing of arsenic from a gallium arsenide substrate during annealing are described in U.S. Pat. No. 4,267,014 which issued on May 12, 1981 to J. E. Davey, A. Christou and H. B. Dietrich. In U.S. Pat. No. 4,267,014 a method for protecting an ion implanted substrate during the annealing process is described by covering the ion implanted layer with a suitable encapsulant such as a germanium, amorphous gallium arsenide, doped gallium arsenide, or gallium aluminum arsenide. The protective capping layer is applied subsequent to the step of ion implantation. After the step of annealing, the capping layer is removed by selective chemical etching.
In U.S. Pat. No. 4,058,413 which issued on Nov. 15, 1977 to B. M. Welch and R. D. Pashley, a method to prevent dissociation of gallium arsenide during the step of annealing was described using a capping layer of aluminum nitride which was sputtered onto the gallium arsenide substrate.
In U.S. Pat. No. 4,330,343 which issued on May 18, 1982 and in U.S. Pat. No. 4,263,605 which issued on Apr. 21, 1982 both to A. Christou and J. E. Davey, a method of forming a low ohmic contact on gallium arsenide material was described by depositing a refractory material such as titanium tungsten (TiW) and ion implanting impurities such as silicon, selenium or germanium through the layer of TiW. The layer of TiW also functions as a capping layer when annealing the implanted structure to activate the implanted ions. Other materials suggested as a refractory material include tantalum, tungsten, platinum, and molybdenum to a thickness of from 400 to 800 Angstroms.
In U.S. Pat. No. 4,354,198 which issued on Oct. 12, 1982 to Rodney T. Hodgson et al., a capping layer of zinc-sulphide is sputtered to a thickness of 500 Angstroms onto a GaAs surface. The GaAs surface is heated with a laser beam which does not directly heat the zinc-sulphide. The zinc-sulphide or group II-VI compound semiconductor acts as a surface passivator to reduce or control the recombination of charge carriers at the surface of GaAs or other group III-V compound semiconductors.
In a paper entitled "New Application of Se-Ge Glasses to Silicon Microfabrication Technology" by H. Nagai et al. and published in Applied Physics Letters, Vol. 28, No. 3, Feb. 1, 1976, pp. 145-147, a film of Se.sub.75 Ge.sub.25 on a silicon substrate was selectively etched by an alkaline solution. A mercury lamp was used as a light source where the wavelength of the lamp is shorter than the absorption edge of Se-Ge. SeGe was used as a photoresist on silicon, silicon dioxide and silicon nitride. The application of Se-Ge glass films to the patterning of layers for silicon device processing and for the fabrication of photo masks were investigated.
In a paper entitled "A Novel Inorganic Photoresist Utilizing Ag Photodoping in Se-Ge Glass Films" by Akira Yoshikawa et al. and published in Applied Physics Letters, Vol. 29, No. 10, Nov. 15, 1976 pp. 667-679, a photoetching procedure is described using an Se-Ge film for positive or negative photographic sensitivity. The negative resist would include the additional process steps of depositing a thin Ag layer over the Se-Ge film prior to photoexposure followed by developing in an acid solution to remove the Ag remaining on the unexposed area. The Se-Ge film would be etched by an alkaline solution.
In a paper entitled "Bilevel High Resolution Photolithographic Technique for Use with Wafers with Stepped and/or Reflecting Surfaces" by K. L. Tai et al. and published in J. Vac. Sci. Technol., 16 (6), Nov./Dec. 1979, pp. 1977-1979, a tri-level system is described comprising Ag.sub.2 Se/GeSe.sub.2 over a thick organic polymer resistor a bi-level system is described comprising GeSe over a thick organic polymer. The polymer may be etched using the GeSe as a mask in an O.sub.2 plasma. The thick polymer provides a flat surface necessary for high resolution and good line-width control over a wafer surface which may have steps raised as high as 1 .mu.m above the plane of the wafer surface.
In a paper entitled "Dry Development of Se-Ge Inorganic Photoresist" by Akira Yoshikawa et al. and published in Appl. Phys. Lett. 36 (1), Jan. 1, 1980, pp. 107-109, a Se-Ge inorganic resist (chalcogenide glass film) is described where plasma etching results in a large etch-rate difference between Ag photodoped and undoped films. A plasma etch rate of undoped to Ag-doped Se.sub.75 Ge.sub.25 films of 370 to 1 was observed with CF.sub.4 as the plasma source gas.
In a paper entitled "Submicron Optical Lithography Using An Inorganic Resist/Polymer Bi-Level Scheme" by K. L. Tai et al. and published in J. Vac. Sci. Technol., Vol. 17, No. 5, Sept./Oct. 1980, pp. 1169-1176, an inorganic photoresist system is described. The inorganic resist consists of two layers, about 100 Angstroms Ag.sub.2 Se on about 2000 Angstroms GeSe. Upon illumination by light, Ag in the Ag.sub.2 Se layer is photodoped into the GeSe layer. Ag doped GeSe is less soluble or insoluble in the developer. A bi-level resist of GeSe over HPR206 polymer is also shown in FIG. 5 over a polysilicon level of a 16K MOS RAM wafer.
It is, therefore, desirable to provide an inorganic photosensitive material amenable to high resolution such as submicron resolution which may be deposited on a gallium arsenide substrate and also function as an ion implant mask.
It is further desirable to provide a new material, germanium selenide (Ge.sub.x Se.sub.1-x) with respect to gallium arsenide which is photosensitive to form an ion implant mask.
It is further desirable to provide a new material to form a capping material on gallium arsenide material during times when ion implanted gallium arsenide is annealed at high temperature.
It is further desirable to use germanium selenide as both an ion implant mask and as a capping layer over gallium arsenide material.
It is further desirable to reduce the number of processing steps in the fabrication of gallium arsenide semiconductor devices.
It is further desirable to provide photosensitive material capable of delineating ion implant masking geometries as small as 0.37 micrometers.